In the present state of the art there essentially exists two types of injectors located at the inlet of combustion chambers:
a first type of injection mode illustrated eg. in document FR-A-2 543 222 involves separately conveying each of two propellants required for the combustion by means of two inlet pipe systems, the channels of one system surrounding the jets of the other system.
This type of system is shown schematically in FIG. 2a. Here, one of the propellants is conveyed from a cavity 1 by a central duct 13 of a tubular injector 70, while the second propellant contained in a second cavity 2, is introduced by lateral holes 15 and travels along the annular space 14 formed by two coaxial tubular elements 11 and 12 before penetrating into the combustion chamber 5. Combustion then occurs, in a known manner, by chemical reaction between the two propellants inside the combustion chamber 5.
A second type of injection mode is employed in the so called "integrated flux" motors.
FIG. 2b shows schematically an example that is more complex than the first one and which includes three separate cavities: two cavities 1,2 for the respective propellants and a cavity 3 for hot gases.
One of the propellants is injected from the first cavity 1 simultaneously through two tubular elements 11 and 16 of two respective injectors 80,90. The other propellants is injected from the second cavity 2 through holes provided in an external tubular element 18 coaxial with tubular element 16. The second propellant is thus injected into the combustion chamber through an annular space 19 defined by coaxial tubular elements 16,18, and mixes with the first propellant transported via the central channel 17. In the case of "integrated flux" motors, a second injection occurs in the same chamber: this is the injection of a mixture of the first propellant introduced by the central channel 13 of injector 80 with the partially burnt hot gases generated by one of the two propelants in one or a plurality of pre=chambers feeding the cavity 3, and introduced by lateral holes 35 through the external tubular element 12 of the injector 80 to be guided across the annular space 14 defined by the tubular elements 11 and 12 towards the combustion chamber.
Document FR-A-2 570 129 shows an embodiment in which a first propellant is brought directly into the combustion chamber by a central tube, while a second propellant is brought simultaneously through an annular space, the two propellants coming from two separate cavities. According to this embodiment, a buffer chamber is further provided inside one of the two first cavities in the injector head to take in the combustion gases repulsed from the combustion chamber through a transfer channel, This third cavity is devised for damping vibrations which could occur in the gas during the combustion process within the combustion chamber. Also, small openings are formed in the wall separating the buffer chamber from the cavity containing reducing propellant so as to allow a scavenging flow to be introduced into the buffer chamber, thereby preventing reactive mixtures from forming within the latter. The buffer chambers formed in the injector head are not connected to any collector so that the combustion products that temporarily enter therein must afterwards necessarily be expulsed into the combustion chamber.
Devices for tapping off hot gases from a combustion chamber, such as those used in the J2S type rocket motors produced by the American company Rocketdyne, are also known. But these gas tapping off devices are complex and costly, and further require a special installation involving specific technologies.